Method and device for selective anodization



March 15, 1965 MAlsSEL 3,240,685

METHOD AND DEVICE FOR SELECTIVE ANODIZATION Filed Feb. 23, 1962 2 Sheets-Sheet 1 FIG. 1

- POWER SUPPLY 28 POWER/ SUPPLY FIG. 2

INVENTOR LEON I. MAISSEL ATTORNEY March 15, 1966 1.. MAISSEL METHOD AND DEVICE FOR SELECTIVE ANODIZATION 2 Sheets-Sheet 2 Filed Feb. 23, 1962 FIG. 3

FIG. 4

FIG. 7

United States Patent 3,240,685 METHOD AND DEVICE FOR SELECTIVE ANODIZATION Leon I. Maissel, Poughkeepsie, N.Y., assignor to International Business Machines Corporation, New York, N.Y., a corporation of New York Filed Feb. 23, 1962, Ser. No. 174,985 17 Claims. (Cl. 20415) The present invention relates to an improvement in the anodization of articles, and more particularly, to a method and a device for the anodization of selective areas of thin metal layers to produce a layer having a pattern of both anodized and unanodized areas.

Anodization of selected areas prior to this invention was accomplished by various masking techniques. These masks or shields are made of nonconductive material. When applied over areas of an article which are not to be anodized, the masks divert current away from these covered areas and thus retard anodization. These techniques while effective to a degree are time consuming and merely add an extra step to the production procedure.

It is an object of this invention to provide a method and a shaped electrode device for selectively anodizing a metal articles which simple, effective and suited to mass production techniques.

It is a further object of this invention to provide a method for forming conductors within a body of dielectric material by a selective anodization procedure.

It is a still further object of this invention to provide a method for producing thin film resistors and capacitors in any desired pattern on a substrate by a selective anodization procedure.

These and other objects are accomplished in accordance with the broad aspects of present invention by utilizing a novel, shaped, anodization electrode system. The shaped electrode system is constructed so as to suppress the growth of the oxide in certain areas. In the areas of a metal article where no oxidation is desired, the shaped electrode system has projections which may be placed closely adjacent to or contiguous to the article. The remaining portions of the shaped electrode adjacent to which anodization is desired are depressed. There are conductive layers on both the projections and at the bottom of the depressions of the shaped electrode. The conductive layer on the surface of the projections is maintained at substantially the potential of the anode, the article to be anodized. The conductive layer on the bottom of the depressions is the cathode of the electrolytic cell. The purpose of the projections of the shaped electrode positioned adjacent to the metal article is to discharge all hydroxyl ions in its immediate vicinity thus suppressing the growth of the metal oxide in these areas. In order to keep the current in the projections from getting too large and also to insure good resolution, the projection surfaces may be placed in actual contact with the article.

The selective anodization technique may be utilized to fabricate thin film components such as resistors, capacitors, parallel conductors, tunnel emission diodes and similar thin film elements. Two shaped electrodes of different surface configuration are used in this procedure. The first electrode has depressions of the desired shape of the area to be anodized. Anodization of the article is accomplished in an electrolytic cell partially through the article. A second shaped electrode having a depression cross-sectional shape other than the first shaped electrode is then positioned over the article to be anodized. Anodization is accomplished through the thickness of the articles area designated to be anodized by the first shaped electrode. Portions of the article can thereby be left unanodized within the dielectric body of the article. The

3,240,685 Patented Mar. 15, 1966 various passive thin film components can be fabricated by choice of appropriate geometry of the shaped electrodes.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.

In the drawings:

FIGURE 1 is a view, partially in section, of a first embodiment of this invention;

FIGURE 2 is a view, partially in section, of a second embodiment of this invention;

FIGURE 3 is a sectional view taken along line 3-3 of FIGURE 1 showing structure of the shaped electrode of this invention in a plane view;

FIGURE 4 is a plane view of thin film resistors that can be made by a method of this invention;

FIGURE 5 is a sectional view of FIGURE 4 taken along line 5-5;

FIGURE 6 is a plane view of thin film capacitors that can be made by a method of this invention; and,

FIGURE 7 is a sectional view of FIGURE 6 taken along line 77.

Similar reference characters are used for similar elements throughout the drawing.

Referring now, more particularly, to FIGURES 1 and 3, there is shown an electrolytic cell 10 with an electrolyte level 11 having positioned therein the article 12 to be anodized and adjacent thereto the shaped electrode SYSI61'111114 of this invention. The article 12 may be a thin film of metal 16 deposited on a glass substrate 18. The shaped electrode system 14 is constructed of an insulating member 20 having openings or depressions 22 therethrough. A conductive layer 24 is applied to one side of the insulating member containing the openings. A base member 26 composed of conductive material is placed on the side of the insulating member 20 containing the openings 22 opposite to the side having the conductive layer 24. A power supply 28 is connected across the base conductive member 26, which is made the cathode of the cell, and the article to be anodized, which is made the anode of the cell. Progress of the anodization process may be observed by use of an ammeter 30 and a voltmeter 32 in the circuit.

FIGURE 2 illustrates a simplified electrical circuit for selective anodization. The conductive layer 24 on the projections of the shaped electrode system 14 is in touching relation to the article 12 to be anodized. Electrical connection to the positive side of the power supply for making the article 12 the anode of the cell is then required only to the conductive layer on the projections, rather than to both the conductive layer on the projections and the article to be anodized as in the FIGURE 1 embodiment. This touching relation of the shaped electrode and the article additionally has the advantage of reducing excessive current in conductive layer 24.

In operation of the FIGURE 1 and FIGURE 2 devices, the metal article 12 and the shaped electrode system 14 are positioned adjacent to one another covered by an appropriate electrolyte within the electrolytic cell 10. The base conductive member 26 is made the cathode of the cell by connecting it electrically to the negative side of power supply 28. The metal article to be anodized is made the anode of the cell by electrically connecting it to the positive terminal of the power supply 28. The conductive layer 24 is maintained at substantially the positive potential of the anode. The metal article may be composed of a valve metal such as aluminum, tantalum, niobium, zirconium, hafnium, tungsten, bismuth, antimony, silicon, germanium, etc. The electrolyte is chosen such that it will not dissolve the oxide as it is formed.

The power supply is turned on and the metal is anodically formed at an approximately constant current density while the voltage across the growing oxide is slowly raised to the desired forming voltage. Anodization is then allowed to continue at this constant voltage while the anodizing current decreases steadily as a more perfect oxide is formed. In the areas where the conductive layer 24 is adjacent to the metal article the growth of oxide is completely suppressed due to the discharge of all hydroxyl ions in its immediate vicinity. In the areas of the article opposite to the depressions of the shaped electrode, hydroxyl ions are discharged at the surface of the metal article and anodization is effected. The anodization of the desired selected areas of the article is thereby accomplished.

Non-aqueous electrolytes are preferred to the aqueous electrolytes. Aqueous solutions tend to produce quantities of oxygen gas at the conductive layer 24 surfaces which reduces resolution. This can be eliminated by mechanical means. However, if non-aqueous electrolyte solutions with anhydrous salts are used, resolution is excellent.

The shaped electrode system may readily be fabricated from a sheet of glass or similar insulator material having sufliciently good heat and chemical resistance properties to withstand subsequent processing. Openings in the glass sheet may be cut by various known techniques to any desired shape. After cutting, the glass sheet is cleaned thoroughly in boiling detergent followed by rinsing and drying of the sheet. The conductive layer 24 is then applied by, for example, spraying or silk screening onto the surface of the sheet surrounding the openings on one side of the dielectric member. This conductive layer must be made of a material which will not oxidize in the anodizing solution. The preferred conductor material platinum may readily be applied to the sheet in the form of a commercially available platinum paste and fired for ten minutes at 650 C. The conductive layer 24 may come to the edge of the opening 22 as illustrated in FIGURE 2 or the immediate regions surrounding the openings may be free of the conductive layer as at windows 25, in FIG- URES 1 and 3. To produce this window 25 which is free of the conductive layer an etching step is used. The conductive layer is protected by use of electroplaters tape and the shaped electrode is placed in a hydrofluoric acid etching solution at room temperature for about 30 minutes, then rinsed and dried. The dielectric member 20, in the window area 25, is etched away to form a shoulder surrounding each opening. A piece of conductive metal foil will serve as the base member cathode 26 and may be held against the dielectric member by clamps.

The invention is further illustrated by the following detailed procedure using tantalum as an example. Tantalum metal was cathode sputtered to a thickness of 100 ohms per square onto a glass substrate. The shaped electrode system 14 and the tantalum deposited article 12 were cleaned in acetone and then held together so that the tantalum deposit and a platinum conductive layer side of the shaped electrode were facing each other and in contact. A piece of tantalum foil was positioned on the side of the shaped electrode opposite to the tantalum article. The pieces were held together lightly by two stainless steel clamps. A 0.5% by Weight solution of anhydrous ammonium acetate in ethylene glycol was used as the electrolyte. The tantalum conductive sheet was made the cathode and the conductive layer attached to the positive side of the power supply. Since the conductive layer was held against the tantalum article to be anodized, the article was at a positive potential and is the anode of the cell. The current was adjusted to 30 milliamps and anodization continued for 15 minutes. The voltage across the growing oxide was raised to and maintained at 100 volts. The anodized tantalum article shows very accurately the pattern of the shaped electrode. The tantalum that was not in contact with the shaped electrode 4. is clear and transparent tantalum oxide. The tantalum that was in contact with the shaped electrode remained in its original metallic form. The demarcation between the anodized and unanodized areas was excellent.

Thin film components such as resistors, capacitors, parallel conductors, tunnel emission diodes and similar thin film elements may be made by an adaptation of the selective anodization technique described above. This method for making articles having at least one conductor layer within a dielectric body by selective anodization of a metal layer can be more readily understood by reference to FIGURES 47.

The method for forming resistors within a dielectric body by selective anodization of a metal layer on a substrate 40 can be more readily understood with reference to FIGURES 4 and 5. A first shaped electrode is provided with depressions having the cross-sectional shape of the layer to be anodized. The conductive areas of the first shaped electrode adjacent to the portions of the metal layer that are designated as the resistor 42 and the resistor contacts 44 are maintained at the positive potential of the anode metal layer. Anodization partially through the thickness of the metal layer is accomplished in the electrolytic cell in the usual way. The first shaped electrode is then removed from the metal layer and a second shaped electrode is positioned adjacent to the metal layer. The conductive areas of the second shaped electrode adjacent to the portions of the metal layer that are designated as resistor contacts 44 are maintained at the positive potential of the anode metal layer. Anodization completely through the thickness of the metal layer in the areas originally designated as the dielectric body by the shape of the first shaped electrode is then accomplished in an electrolytic cell in the usual manner. Anodization during the use of the second electrode proceeds partially through the thickness of the area 46 leaving a resistive metal film between the substrate and the anodized area. The thickness of this resistance film may be, of course, regulated by the time and voltage used during the second anodization process to produce a range of possible resistances.

Referring now to FIGURES 6 and 7, a thin film capacitor may be formed by depositing a metal layer on a substrate 50 wherein at least one selected portion 52 of the layer is thicker than other areas. The first shaped electrode is designed to have its areas adjacent to the metal layer to be anodized that are designated as the bottom electrode 52 and the bottom contact 54 of the capacitor at the positive potential of the anode metal layer. The remaining area of the shaped electrode is at the cathode potential and spaced at a distance from the metal layer. Anodization partially through the thickness of the metal layer is effected in the usual manner in an electrolytic cell. The first shaped electrode is then removed from the metal layer and a second shaped electrode is positioned adjacent to the partially anodized metal layer. The areas of the shaped electrode adjacent to the portions of the metal layer designated as the bottom electrical contacts 54 are maintained at a positive potential of the metal layer. Complete anodization in the electrolytic cell is then continued for a time suflicient to anodize the metal layer through its thickness in areas originally designated to be anodized by the first shaped electrode. This leaves the areas of the capacitor designated bottom contacts in their metallic condition. The surface 56 of the bottom electrode of the capacitor has an anodized coating thereover. A counter electrode 58 is then deposited on the opposite side of the bottom electrode in the usual manner, such as by vacuum evaporation, to form a capacitor.

The invention provides a method and a shaped electrode device for selectively anodizing articles composed of any of the usual metals that can be anodized using standard electrolytes. The shaped electrode device is readily fabricated and the anodization method can be adapted to large scale production. A metal layer may thus be anodized using the shaped electrode device in any desired pattern of anodized and unanodized areas. Furthermore, electrical components may be formed in the desired pattern on a metal layer and then covered with an anodized layer for insulation purposes.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. In a shaped electrode for selectively anodizing an article, said electrode comprising:

a member composed of insulating material having at least one opening therethrough;

a conductive layer on one of the sides of said member containing said opening;

a member of conductive material positioned on the side of said insulating member containing the opening opposite to the side having said conductive layer thereon;

said conductive layer and member being composed of a material that will remain substantially unoxidized in the anodizing solution;

means for connecting said conductive layer to a source of positive potential;

and means for connecting said member of conducting material to a source of negative potential.

2. In a shaped electrode for selectively anodizing a fiat metal article, said electrode comprising:

a base member composed of insulating material having a plurality of laterally spaced flat-tipped projections extending therefrom, the surface of said fiat-tipped projections defining a single plane;

a conductive layer on each of said projection tips;

a conductive layer on each of the portions of the base member without the said projections said conducting layers being composed of a material that will remain substantially unoxidized in the anodizing solution;

means for connecting said projection tip conductive layers to a source of positive potential;

and means for connecting said base member layers to a source of negative potential.

3. In a shaped electrode for selectively anodizing a metal article, said electrode comprising:

an insulating member having at least one depression in its surface;

the cross-section of said depression being the shape of the area of said article to be anodized;

a conductive layer on the bottom of said depression;

a conductive layer on the said surface of said insulating member;

said conducting layers being composed of a material that will remain substantially unoxidized in the anodizing solution;

means for connecting said surface conductive layer to a source of positive potential;

and means for connecting said depression conductive layer to a source of negative potential,

4. In a shaped electrode for selectively anodizing a metal article, said electrode comprising:

an insulating member having at least one depression in its surface;

the cross-section of said depression being the shape of the area of said article to be anodized;

a conductive layer on the bottom of said depression;

a conductive layer on the said surface of said insulating member;

said conducting layers being composed of a material that will remain substantially unoxidized in the anodizing solution;

the said surface of said insulating member being free 6 of said surface conductive layer in the region surrounding said depression; means for connecting said surface conductive layer to a source of positive potential; 5 and means for connecting said depression conductive layer to a source of negative potential.

5. In a shaped electrode for selectively anodizing a metal article, said electrode comprising:

a member composed of insulating material having a plurality of openings therethrough;

a conductive layer on one of the sides of said member containing said openings;

the said side of the insulating member having said conductive layer thereon being free of said conductive layer in the immediate region surrounding the said openings;

a member of conductive material positioned on the side of said insulating member containing said openings opposite to the said side having said conductive layer thereon;

said conducting layers being composed of a material that will remain substantially unoxidized in the anodizing solution;

means for connecting said conductive layer and conductive member to sources of electrical potential.

6. A method of anodizing an article in selected areas comprising:

passing an electric current through an electrolytic cell containing an anodizing electrolyte with said article immersed in the electrolyte forming the anode electrode of the cell;

maintaining selected areas of an electrode system at substantially the positive potential of the said anode and positioned adjacent to said anode;

electrically insulating said selected areas from the remaining area of said electrode system;

and maintaining the said remaining area of said electrode system at a negative potential and spaced at a distance from the said anode, whereby the growth of an oxide on said anode is suppressed in the areas where the electrode system is held at a positive potential and promoted where the electrode system is held at a negative potential.

7. A method for anodizing a metal article in selected areas comprising:

passing an electric current through an electrolytic cell containing an anodizing electrolyte with said article immersed in the electrolyte forming the anode electrode of the cell;

malntarnlng selected areas of an electrode system at the positive potential of the said anode and positioned contiguous to the said anode; electrically insulating said selected areas from the remaining area of said electrode system;

and maintaining the said remaining area of said electrode system at a negative potential and spaced at a distance from the said anode, whereby the growth of an oxide on the said anode is suppressed in the areas where the electrode system is held at a positive potential and promoted where the electrode system is held at a negative potential.

8. A method for anodizing a metal article at selected areas in an electrolytic cell comprising:

positioning a shaped electrode having at least one depression therein adjacent to said metal article within the electrolyte of said cell;

making the bottom of the depression the cathode of the said cell;

electrically insulating the said cathode of the said cell from the remaining portions of said shaped electrode;

making the said article the anode of the said cell;

maintaining the surface of said shaped electrode adja- 27 cent to said article at substantially the potential of said anode;

and passing a current between said cathode and said anode.

9. A method for anodizing a metal article at selected areas in an electrolytic cell comprising:

positioning a shaped electrode having a plurality of projections in close proximity to the said metal article within the electrolyte of said cell;

making the bottom of the depressions between the said projections the cathode of the said cell;

electrically insulating the said cathode of the said cell from the remaining portions of said shaped electrode; making the said article the anode of the said'cell; maintaining the tips of the said projections adjacent to said article at substantially the potential of the said anode;

and passing current between said cathode and said anode.

10. A method for anodizing a metal article at selected areas in an electrolytic cell comprising:

positioning in the electrolyte of said cell a shaped electrode having at least one depression therein with the surface of the electrode in touching relation with the said metal article and the depressions spaced at a distance therefrom;

making the bottoms of the said depressions the cathode of the said cell;

electrically insulating the said cathode of the said cell from the remaining portions of said shaped electrode; making the said article the anode of the said cell; maintaining the surface of the said electrode adjacent to said article at substantially the potential of said anode, said electrolytic cell containing an electrolyte in which the oxide formed by anodization is insoluble;

and passing current between said cathode and said anode.

11. A method for anodizing a metal article at selected areas in an electrolytic cell comprising:

positioning in the electrolyte of said cell a shaped electrode having a plurality of projections and depressions with the tips of the projections contiguous to the said metal article and the depressions spaced at a distance therefrom;

making the bottom of said depressions the cathode of the said cell; electrically insulating the said cathode of the said cell from the remaining portions of said shaped electrode;

making the said article the anode of the said cell by applying a positive potential to the anode through the said continguous projection tips, said electrolytic cell containing an electrolyte in which :the oxide foumed by anodization is insoluble;

and passing current between said cathode and said anode.

12. A method for making articles having at least one conductor layer within dielectric body by selective anodization of a metal layer comprising:

passing a first electric current through an electrolytic cell containing an anodizing electrolyte with said metal layer immersed in the electrolyte forming the anode of the cell;

maintaining areas of a first shaped electrode which are positioned adjacent to areas of said metal layer that are designated as the said conductor layer and the electrical cantacts at the positive potential of the said anode;

electrically insulating said areas from the remaining area of said first shaped electrode;

maintaining the said remaining area of said first shaped electrode at a negative potential and spaced at a distance from the said anode;

turning off said first current before anodization of the metal layer is complete;

passing a second electric current through an electrolytic cell containing an anodizing electrolyte with said metal layer immersed in the electrolyte forming the anode of the cell;

maintaining areas of a second shaped electrode which are positioned adjacent to areas of said metal layer that are designated as the electrical contacts at the positve potential of said anode;

electrically insulating said areas from the remaining area of said second shaped electrode;

maintaining the said remaining area of said second shaped electrode at a negative potential and spaced at a distance from the said anode;

and turning off said second current when the said metal layer has been anodized through its thickness in the areas designated as the said dielectric body.

13. A method of forming resistors within a dielectric body by selective anodization of a metal layer comprising:

passing a first electric current through an electrolytic cell containing an anodizing electrolyte with said metal layer immersed in the electrolyte forming the anode of the cell;

maintaining areas of a first shaped electrode which are positioned adjacent to areas of said metal layer that are designated as the resistors and the resistor contacts at the positive potential of the said anode;

electrically insulating said areas from the remaining area of said first shaped electrode;

maintaining the said remaining area of said first shaped electrode at a negative potential and spaced at a distance from said anode;

turning off said first current before anodization of the metal layer is complete;

passing a second electric current through an electrolytic cell containing an anodizing electrolyte with said metal layer immersed in the electrolyte forming the anode of the cell;

maintaining areas of a second shaped electrode which are positioned adjacent to areas of said metal layer that are designated as the resistor contacts at the positive potential of said anode;

electrically insulating said areas from the remaining area of said second shaped electrode;

maintaining the said remaining area of said second shaped electrode at a negative potential and spaced at -a distance from said anode;

and turning off said second current when the said metal layer has been anodized through its thickness in the areas designated as the said dielectric body.

14. A method of forming capacitors Within a dielectric body by selective anodization comprising:

depositing a metal layer on a substrate wherein at least one selected portion of said metal layer is thicker than other areas;

passing a first electric current through an electrolytic cell containing an anodizing electrolyte with said metal layer immersed in the electrolyte forming the anode of the cell;

maintaining areas of a first shaped electrode which are positioned adjacent to areas of said metal layer that are designated as the bottom electrode of the said capacitor and the bottom contact at the positive potential of said anode;

electrically insulating said areas from the remaining area of said first shaped electrode;

maintaining the said remaining area of said first shaped electrode at a negative potential and spaced at a distance from said anode;

turning off said first current before anodization of the metal layer is complete;

passing a second electric current through an elect lytic cell containing an anodizing electrolyte with said metal layer immersed in the electrolyte forming the anode of the cell; maintaining areas of a second shaped electrode which are positioned adjacent to areas of said metal layer that are designated as the bottom contact of the said capacitor at the positive potential of said anode; electrically insulating said areas from the remaining area of said second shaped electrode; maintaining the said remaining area of said second shaped electrode at a negative potential and spaced at a distance from the said anode; turning ea said second current when the said metal layer has been anodized through its thickness in the areas designated as the said dielectric body; and depositing a counter electrode on the opposite side of the anodized metal from the bottom electrode. 15. A method for making articles having at least one conductor layer within a dielectric body by selective anodization of a metal layer comprising:

positioning a first shaped electrode having at least one depression adjacent to the said metal layer within the electrolyte of an electrolytic cell; the crosssectional shape of said depression being the desired shape of the area of the said metal layer to be anodized; making the bottom of the said depression the cathode of the said cell; electrically insulating the said cathode of the said cell from remaining portions of said first shaped elctrode; making the said article the anode of the said cell; maintaining the surface area of said shaped electrode adjacent to said metal layer at substantially the potential of said anode; passing a first electric current between said cathode and said anode for a period of time sufiicient t anodize partially through the said metal layer; replacing said first shaped electrode with a second shaped electrode having at least one depression adjacent to said metal layer within the electrolyte of said electrolytic cell; the cross-section of the depression of said second shaped electrode being other than the cross-section of the depression of said first shaped electrode; making the bottom of the said depression the cathode of the said cell; electrically insulating the said cathode of the said cell from the remaining portions of said second shaped electrode; making the said article the anode of the said cell; maintaining the surface area of said second shaped electrode adjacent to said metal layer at substantially the potential of the said anode; and passing a second electric current between said cathode and said anode for a time suflicient to anodize through the thickness of said metal layer areas designated to be anodized by said first shaped electrode. 16. A method for making resistors within a dielectric body by selective anodization of a metal layer comprising: positioning a first shaped electrode having at least one depression adjacent to the said metal layers within the electrolyte of an electrolytic cell; the cross-sectional shape of said depression being the desired shape of the area of the said metal layer to be anodized between the areas designated as the said i resistor and the resistor contacts; making the bottom of the said depression the cathode I of the said cell;

electrically insulating the said cathode of the said cell from the remaining portions of said first shaped electrode; making the said article the anode of the said cell; maintaining the surface area of said shaped electrode adjacent to said metal layer at substantially the potential of the said anode;

passing a first electric current between said cathode and said anode for a period of time sufficient to anodize partially through the said metal layer;

replacing said first shaped electrode with a second shaped electrode having at least one depression adjacent to said metal layer within the electrolyte of said electrolytic cell;

the cross-section of the depression of said second shaped electrode being the desired shape of the said metal layer to be anodized surrounding the areas designated as the resistor contacts;

making the bottom of the said depression the cathode of the said cell;

electrically insulating the said cathode of the said cell from the remaining portions of said second shaped electrode;

making the said article the anode of the said cell;

maintaining the surface area of said second shaped electrode adjacent to said metal layer at substantially the potential of the said anode;

and passing a second electric current between said cathode and said anode for a time sutficient to anodize through the thickness of said metal layer areas designated to be anodized by said first shaped electrode.

17. A method for making capacitors within a dielectric body by selective anodization of a metal layer having at least one selected area thicker than other areas comprising:

positioning a first shaped electrode having at least one depression adjacent to the said metal layer within the electrolyte of an electrolytic cell;

the cross-sectional shape of said depression being the desired shape of the area of the said metal layer to be anodized between the areas designated as the bottom electrode of the said capacitor and the bottom contact;

making the-bottom of the said depression the cathode of the said cell;

electrically insulating the said cathode of the said cell from the remaining portions of said first shaped electrode;

making the said article the anode of the said cell;

maintaining the surface area of said shaped electrode adjacent to said metal layer at substantially the potential of the said anode;

passing a first electric current between said cathode and said anode for a period of time sufiicient to anodize partially through the said metal layer;

replacing said first shaped electrode with a second shaped electrode having at least one depression adjacent to said metal layer within the electrolyte of said electrolytic cell;

the cross-section of the depression of said second shaped electrode being the desired shape of the said metal layer to be anodized surrounding the areas designated as the capacitor bottom contacts;

making the bottom of said depression the cathode of the said cell;

electrically insulating the said cathode of the said cell from the remaining portions of said second shaped electrode;

making the said article the anode of said cell;

maintaining the surface area of said second shaped electrode adjacent to said metal layer at substantially the potential of the said anode;

and passing a second electric current between said cathode and said anode for a time sufiicient to anodize through the thickness of said metal layer areas designated to be anodized by said first shaped electrode;

3,240,685 1 ll 1 2 and depositing a counter electrode on the opposite side 2,739,935 3/1956 Kehl et a1 204-224 of the anodized metal from the bottom electrode. 3,008,892 11/1961 Owen 204-224 JOHN A. MACK, Primary Examiner.

5 JOSEPH REBOLD, Examiner.

References Cited by the Examiner UNITED STATES PATENTS 2,306,082 12/ 1942 Prest 204224 

1. IN A SHAPED ELECTRODE FOR SELECTIVELY ANODIZING AN ARTICLE, SAID ELECTRODE COMPRISING: A MEMBER COMPOSED OF INSULATING MATERIAL HAVING AT LEAST ONE OPENING THERETHROUGH; A CONDUCTIVE LAYER ON ONE OF THE SIDES OF SAID MEMBER CONTAINING SAID OPENING; A MEMBER OF CONDUCTIVE MATERIAL POSITIONED ON THE SIDE OF SAID INSULATING MEMBER CONTAINING THE OPENING OPPOSITE TO THE SIDE HAVING SAID CONDUCTIVE LAYER THEREON; SAID CONDUCTIVE LAYER AND MEMBER BEING COMPOSED OF A MATERIAL THAT WILL REMAIN SUBSTANTIALLY UNOXIDIZED IN THE ANODIZING SOLUTION; MEANS FOR CONNECTING SAID CONDUCTIVE LAYER TO A SOURCE OF POSITIVE POTENTIAL; AND MEANS FOR CONNECTING SAID MEMBER OF CONDUCTING MATERIAL TO A SOURCE OF NEGATIVE POTENTIAL. 